Heat exchanger coils of the type commonly used in air conditioning units for both domestic and commercial applications are formed by a plurality of parallel tubes having a number of fins mounted thereon for dissipating the heat contained in the fluid moving through the tubes. The size and density of the fins principally determine the rate of heat dissipation provided by the coil. In order to increase the effectiveness of coil, the fins are lanced to form tabs in the individual fins which increases turbulence of the cooling air and correspondingly increases the heat loss. However, the lancing of fins results in a weakened coil. These lanced fins are referred to as "enhanced fins" and enables less fins to be used due to the increased efficiency, but provides a fin of reduced strength providing structural problems, especially during the bending of the coil into a desired configuration.
The bending of the coil which is constructed in a linear sheet is necessary to enable the coil to be fitted into a housing to provide a compact unit as possible. These straight, flat coils usually are formed with one, two or three generally 90.degree. bends to form an "L"-shaped, "U"-shaped or box-shaped coil for installation in the housing of the air conditioning unit.
Existing bending apparatus has been found to be inefficient to form the bend on such enchanced finned coils since the fins in the area of the bends are distorted, especially on the inside of the bends which stops the air flow through the corner of the coil reducing the cooling efficiency of the coil. Therefore, the need exists for an improved coil bending apparatus and method for bending enhanced finned coils without damaging the fins in the bend area.
Examples of known prior art coil bending apparatus and methods are shown in U.S. Pat. Nos. 3,443,296 and 3,597,956 in which the coil is bent by a pair of pivotally mounted grippers located on opposite side of the bend area. U.S. Pat. No. 4,173,998 discloses another coil forming method and apparatus in which the bending action is compensated for during the initial construction of the coil so that the coil after being bent, will have the desired bend without damaging the fins.
U.S. Pat. No. 3,831,420 is believed to be one of the most pertinent prior art with respect to our invention in that it discloses a method of bending a heat exchanger coil in which an axial force is applied to the rear of the coil to bring the material of the tubes to a critical state just preceding the loss of longitudinal stability of the tubes as the tube is bent by moving it along a surface which has the final shape into which the coil is to be bent.
Another known coil bending apparatus and method of which the present invention is an improvement utilizes a flat, horizontal table along which the exchanger coil is moved by pneumatic cylinders for advancing a forward end of the coil at a constant speed onto an anvil where it is clamped by a clamping pad, after which the clamping pad and anvil which are mounted on a spindle, are rotated through a predetermined angle for imparting the desired bend angle on the coil. Another portion of the coil located just rearwardly of the anvil and clamping pad prevents the upward movement of the coil from the support table during the rotation of the anvil and clamping pad. Although this apparatus and method forms satisfactory bends for many finned coils, it has not proved entirely satisfactory for bending enhanced finned coils due to the less structural strength provided by the lanced fins.
Therefore, the need exists for an improved heat exchanger coil bending apparatus and method which eliminates the difficulties present in existing apparatus and methods to provide a bend in a heat exchanger coil, especially a coil having enhanced or lanced fins.